Hi,
In this case, you need to share the authentication cookie across domains - mercola.com and login.mercola.com. Please see www.15seconds.com/issue/971108.htm.
However, the user you are authenticating must exist in Kentico
CMS database.
If the user does not exist in the Kentico CMS database, you need to use
custom security handler to authenticate the user against external DB.
Best regards,
Juraj Ondrus